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Measuring Physical Properties at the Surface of a Comet Nucleus

By Andrew J. Ball


The European Space Agency’s cornerstone mission Rosetta is due for launch in\ud January 2003. It will perform a rendezvous with comet 46P/Wirtanen beyond 3 AU and,\ud following an initial mapping phase, deploy a lander to a selected site on the nucleus\ud surface. The Rosetta Lander will provide unprecedented access to cometary material.\ud Some of the most uncertain characteristics of the nucleus material are physical properties\ud such as its density, the structure of the surface layers and its mechanical strength.\ud MUPUS (Multi-Purpose Sensors for Surface and Sub-Surface Science) is one of the\ud experiment packages selected for the Lander payload which will address certain physical\ud properties and their evolution with time. This thesis focuses on the in situ measurement of\ud the density of the surface layers by a radiation densitometer incorporated into the MUPUS\ud thermal probe, and on the penetrometry measurements to be performed by an\ud accelerometer mounted in the Lander’s anchoring harpoon.\ud A concept for incorporation of a gamma ray attenuation densitometer into the\ud thermal probe is presented and explored. A 137Cs radioisotope source will be mounted near\ud the tip of the probe and semiconductor radiation detectors situated at the top of the probe\ud will monitor the transmitted count rate during probe insertion, as the intervening material\ud attenuates the radiation. Preliminary experiments to evaluate cadmium telluride (CdTe)\ud detectors for this purpose are presented, as well as results from a specially-developed\ud Monte Carlo computer code designed to model the absorption and scattering of photons in\ud bulk material.\ud Also presented is a control algorithm to dynamically re-budget the integration time\ud and depth resolution of the instrument as it is inserted by the hammering mechanism. This\ud is required due to: a) the wide range of possible densities the instrument may encounter, b)\ud the variation vs. depth of required integration time, and c) the limited time in which the\ud measurement must be performed. For lower than nominal densities, integration time may\ud be wasted when it could be used to improve the accuracy and depth resolution. For higher\ud densities the integration time at particular depths may not be sufficient to obtain acceptable\ud accuracy; in this case some depth resolution could be sacrificed to improve the accuracy.\ud The proposed algorithm uses the density measured at each point to update the time budget\ud and depth resolution for the remaining stages of penetration.\ud Although the use of the gamma ray backscatter type of densitometer was eventually\ud rejected in favour of the aforementioned attenuation technique, investigation of the\ud backscatter technique resulted in an extension to the Single Scattering Model– an analytic\ud approximation of its operation. This extended model adds to our understanding of these\ud devices' response to spatial inhomogeneity.\ud Calculations show that anchoring of the Lander is necessary to avoid possible\ud ejection from the nucleus by gas drag in the case of a landing in an active area. The use of\ud the Lander’s anchoring harpoon to perform penetrometry measurements is reported,\ud including the results of preliminary experiments and techniques for analysing the\ud accelerometry data. It is shown that layers with distinctly different strengths may be\ud identified, and that the mean deviatoric stress– a strength parameter– may be constrained to\ud within a factor of about 2.2. This would be a significant improvement on current estimates,\ud which vary by several orders of magnitude.\ud Together with other investigations on the Rosetta mission the densitometry and\ud penetrometry measurements will serve to constrain models of the physical state and\ud evolution of the cometary material found at the landing site. In particular both instruments\ud are sensitive to near-surface layering, which may be expected from theoretical models of\ud cometary activity

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